Morrison’s Space in the FAST Examination

Anatomical Importance

• The Morrison’s space (hepatorenal recess) is the peritoneal compartment located between the right hepatic lobe and the right kidney and must be visualized in every FAST exam to detect free intraperitoneal fluid. 1

FAST Protocol Requirements

• The FAST protocol mandates evaluation of at least three anatomic windows: the right upper quadrant (Morrison’s space), the left upper quadrant (splenorenal recess), and the pelvic pouch of Douglas. 1

Diagnostic Performance

• In blunt abdominal trauma, FAST identifies intraperitoneal fluid with a sensitivity ranging from 68 % to 91 % and excellent specificity. 1
• Among hypotensive trauma patients, FAST achieves 100 % sensitivity and specificity for detecting hemoperitoneum. 1
• A minimum of approximately 500 mL of free fluid is required for reliable ultrasound detection in Morrison’s space. 4

Technical Aspects of Visualization

• Use a curvilinear abdominal transducer (2–5 MHz) placed subcostally on the right, aligned with the mid‑axillary line, with the probe marker oriented cranially. 5
• Acquire images in both longitudinal and transverse planes to ensure complete assessment. 6
• Position the transducer parallel to the intercostal spaces to facilitate optimal visualization of Morrison’s space. 6

Sonographic Appearance of Pathology

• Free fluid appears as an anechoic or hypoechoic stripe between liver and kidney (“crescent sign”).
• Fluid containing internal echoes suggests blood, which is especially concerning in the context of trauma or a ruptured ectopic pregnancy. 7

Clinical Implications

Hemodynamically Unstable Patients

• Detection of free fluid in Morrison’s space in unstable patients supports immediate decision‑making for exploratory laparotomy. 1

Hemodynamically Stable Patients

• In stable patients with Morrison’s‑space fluid, a contrast‑enhanced CT of the abdomen and pelvis is recommended for definitive evaluation. 4

Extent of Fluid and Surgical Need

• Presence of fluid in all three FAST windows (abundant hemoperitoneum) is associated with a 61 % rate of appropriate laparotomies. 4
• The volume of free fluid correlates with the likelihood of requiring laparotomy; multiple positive FAST sites indicate a strong indication for operative intervention. 4

Limitations

False‑Negative Findings

• A negative FAST does not exclude intra‑abdominal injury, particularly intestinal or mesenteric lesions (observed in 16 of 30 false‑negative cases). 1
• Early or slow bleeding may remain undetected because at least 500 mL of fluid is needed for sonographic visualization. 4

False‑Positive Findings

• Perirenal fat can mimic hemoperitoneum on ultrasound. 4
• Fluid within the stomach or bowel may be misinterpreted as free intraperitoneal fluid. 4
• Non‑traumatic collections such as ascites can be erroneously attributed to trauma. 4
• Clotted blood may resemble soft‑tissue structures and be overlooked. 4

Management Integration

• Unstable patients with Morrison’s‑space fluid: Initiate immediate resuscitation (IV access, fluids, blood products) followed by urgent surgical exploration. 4
• Stable patients with Morrison’s‑space fluid: Proceed to contrast‑enhanced CT for comprehensive assessment before deciding on operative management. 4